The present invention relates to the field of hydraulic servo valves, particularly to two-stage pressure control type servo valves utilized in braking systems which incorporate anti-skid controls. Such valves have a history of degrading due to hydraulic erosion of the first stage when the valve is subjected to full system pressure when it is not in use and/or is delivering at low pressures relative to system pressure; i.e. there is a high pressure drop at low flow levels.
One known solution to this erosion problem is to shut off system pressure to the valve when it is not in use. However, doing so manually has been found unacceptable because of the extra time and attention required to do so, particularly when use requirements occur suddenly, unexpectedly or when the operator's efforts and attention are devoted to more demanding requirements. Therefore prior art has provided pressure turn on and off automatically in various ways as exemplified by U.S. Pat. No. 4,003,400 assigned to The Boeing Company. Pressure flow to this valve is turned on and off in response to the rate of flow of fluid through the valve. This technique is not applicable to anti-skid valves since they are predominantly for pressure control and flow is limited and incidental. Another known system employs a hydraulically actuated on-off valve with the control connected to a hydraulic system which is pressurized in order to extend the landing gear of an airplane. The on-off valve feeds pressure to the anti-skid valve in response to the pressurization of the gear extension system. This system is not satisfactory for two main reasons. First, the anti-skid valve is subject to system pressure for much more time than that during which the brakes function. Second, and more important, if the hydraulic system which extends the gear should fail, the gear can be extended by emergency means without hydraulic pressure and the anti-skid system would be inoperative without this pressure to actuate the on-off valve to the anti-skid valve. This problem of loss of anti-skid control because of loss of hydraulic supply to the first stage of the anti-skid valve can occur with whatever source, not just with the landing gear extend source. Another problem in known systems is that failure of the valve controlling pressure to the first stage will also prevent application of pressure to the first stage when needed and thus prevent anti-skid control.
It is further known that aircraft braking systems typically employ anti-skid systems to prevent tire skidding during heavy braking. These systems control brake pressure by means of an electrically controlled anti-skid valve which reduces or completely releases brake pressure when a skid is detected and then allows braking pressure to be reapplied when it has been detected that the tire/ground friction has spun the wheel back up.
In most aircraft anti-skid systems these valves are two-stage pressure control valves. The first stage typically employs an electro-hydraulic torque motor to convert the electrical brake pressure command signal to a low power hydraulic pressure. The second stage typically employs a hydraulic metering spool which directly controls the brake pressure in response to the low power hydraulic pressure from the first stage.
Although such anti-skid valves utilize the hydraulic pressure metered by the pilot's pedal operated brake metering valve to power both the first and second stages of the anti-skid valve, the type of anti-skid valve in accordance with the present embodiment of the invention utilizes hydraulic system supply pressure, which remains relatively constant at all times, to power the first stage, and meters the pilot's metered pressure, which varies according to the level of braking commanded by the pilot, to the second stage.
The pressure embodiment offers advantages to the anti-skid system in that significant improvements in anti-skid control are allowed whenever the pilot's metered pressure is varied. A problem with the aforementioned type valves is that when the valve is not in use, the quiescent flow from the system supply pressure through the first stage torque motor tends to erode the first stage, thereby degrading its performance.
A means to solve this problem includes the insertion of an electrically operated shutoff valve in the hydraulic supply line to the first stage. This valve is then coupled to a switch in the aircraft which detects landing gear extension so that hydraulic power is applied to the first stage only when the landing gear is extended. This design has problems in that landing gear operated shutoff valve adds significantly to the complexity of the anti-skid system, can allow erosion of the anti-skid valve first stage whenever the landing gear is extended, and could have a tendency toward failure in the closed position causing loss of braking capability or loss of anti-skid protection. An embodiment of the present invention comprises a shutoff valve in the supply line to the anti-skid valve first stage which overcomes the problems of prior shutoff valves. The present embodiment of the invention utilizes a valve which is a hydraulically operated, spring biased shutoff valve which is actuated by the pilot's metered braking pressure so that the valve opens and allows hydraulic system pressure to power the anti-skid valve first stage whenever the pilot meters braking pressure, and then closes to shut off the first stage when the pilot releases braking pressure. A hydraulic check valve is located between the pilot's metered pressure and the first stage supply pressure so that, should the shutoff valve fail closed, the pilot's metered pressure can power the anti-skid valve first stage, yet, when the shutoff valve opens, the system supply pressure cannot be ported to the brakes. This check valve insures that the brakes can be applied and anti-skid control retained even when the aforementioned shutoff valve fails in the closed position.
The significant contribution and consequent inherent advantages of this valve configuration include:
(1) the valve is simple and requires no interface with other aircraft systems such as a landing gear extension system,
(2) the valve only opens when the pilot meters pressure, instead of whenever the landing gear is extended, so that first stage erosion of the anti-skid valve is minimized, and
(3) the check valve of the present system embodiment prevents failures which prevent braking or cause loss of anti-skid protection.